A major part of the investigation is based on determining the action spectra; that is identification of the wavelengths of ultraviolet light, visible and infrared radiation and the minimum intensity of radiation that will provoke changes in the skin. This is determined by irradiating the skin with narrow wavebands usually 4 nm. wide of measured intensity. The light source used is either a 2.5 KW xenon or xenon mercury lamp; monochromatic radiation is obtained with a prism grating monochromator, designed to reduce stray radiation but give sufficiently high output for irradiating patients within a relatively short time. The investigator has studied a variety of photodermatoses, photoallergy to topical chemicals, Lupus Erythematosus, Xeroderma Pigmentosa, Polymorphous light eruption, various forms of porphyria and solar urticarla. Biological systems irradiated include selective killing of T lymphocytes with preservation of B lymphocytes with ultraviolet radiation. Hepatic pathology of Erythropoietic Protoporphyria and comparisons with animal experimental models.